On Thu, 24 Oct 2024 at 20:34, Jens Maurer via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
>
> On 24/10/2024 20.03, Paul Caprioli via Std-Proposals wrote:
> > My colleagues and I are looking into the accuracy of complex
> mathematical operations/functions with different compilers/libraries.
> > The first version of our paper is available at:
> https://inria.hal.science/hal-04714173
> >
> > In the Numerics [complex.transcendentals] section of the C++ standard,
> some functions (e.g., acos) have a remark, "Behaves the same as the C
> function cacos.
> > See also: ISO/IEC 9899:2018." Others (e.g., log) have a note, "The
> semantics of this function are intended to be the same in C++ as they are
> for clog in C."
> > Others (e.g., cos) have neither.
> >
> > Are these differences intentional and meaningful? I would appreciate
> any pointers or help in learning more.
>
> These words probably haven't been touched for three decades.
>

Some of them, yes.

cos(const complex<T>&) was present in C++98, and there was no ccos in ISO C
at the time, so nothing that it could refer to. ccos was added to C99.

acos(const complex<T>&) was not present in C++98, it was added in C++11. By
that time, C had standardized cacos, so acos(complex<T>) could refer to
cacos. It looks like when the new complex functions were added to C++11,
somebody took the opportunity to add references to the corresponding
functions in C99.

log(const complex<T>&) was in C++98, but like the cos/ccos case above, it
couldn't refer to clog because that wasn't in C. The note about the
semantics being intended to be the same as C was added by the defect report
https://cplusplus.github.io/LWG/issue2597 which also removed a
contradictory requirement. So that was clarifying a specific property that
had been previously specified differently. That's why it's not in the same
form as the cross-references for the "new" functions added in C++11.

So the differences are because the wording was added and/or changed at
different times, by different people. I don't think that is intended to
convey any different meaning though. As Jens said, those functions should
all be consistent with the corresponding ones in C.




>
> My guess is that we want "the same" behavior as in C for all of those
> functions
> (e.g. regarding branch cuts), and the differences in note phrasing are
> accidental.
>
> I would guess that all C++ implementations defer to the corresponding C
> functions
> for those trigonometrics.
>
>
> Quote from the paper, section 3:
>
> "The following table shows the results for the complex bivariate
> operations in single precision."
>
> What are the "results"? It would be good to repeat what the numbers mean.
>
> Jens
>
>
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